The Angle of Attack
Every wing has a limit to how slowly it can fly, and that limit is based on its angle of attack. Whenever a plane flies too slowly for the wing’s angle of attack to produce lift, the wing “stalls,” and the plane quickly starts descending.
Sometimes you hear news reports about an airplane “stalling.” Those reports usually don’t have anything to do with the engine stalling as can happen sometimes to a car on a hot day. They usually are referring to a wing stall. If a wing stall happens at an altitude high enough for the pilot to recover, the only result is a shaken-up pilot. But if it happens at a low altitude, such as during an approach to landing, the plane often hits the ground before it can recover enough speed to keep flying. Those kinds of accidents make up most of the accidents on approach to landing. Flight instructors spend a lot of time drilling their students on maintaining safe speed during the approach to landing.